Communication between down hole sensors and the surface has long been practiced in the hydrocarbon recovery industry. Long-range data signal transmission is, for example, an integral part of techniques such as Measurement-While-Drilling (MWD) and Logging-While-Drilling (LWD). Data signals have been transmitted via various carriers such as electromagnetic radiation transmitted through the ground formation, electrical transmission transmitted through an insulated conductor, pressure pulses propagated through the drilling mud, and acoustic waves propagated through the metal drill string. Each of these methods is associated with varying degrees of signal attenuation and ambient noise. There are also difficulties associated with high operating temperatures and compatibility with standard drilling procedures.
The most commercially successful of these methods has been transmission of information by pressure pulses in the drilling mud. However, attenuation mechanisms in the mud limit the effective transmission rate to less than 10 bits/sec for useful depths and mud types, though higher rates have been achieved in laboratory tests. Additionally, conventional mud pulse telemetry fails during drilling with highly compressible fluids such as gassified muds and foams. These fluids are finding an increasing market in underbalanced drilling, but reliably maintaining under-balance requires real-time monitoring of down hole annular pressure and hence high data transmission rates.
An alternative is to use axial or torsional waves propagated in the drill string as a means of carrying data. Drumeller and Knudsen (D. S. Drumheller and S. D. Knudsen, J. Acoust. Soc., Vol. 97(4), April 1995, 2116-2125) provide a useful discussion of the propagation of elastic waves in drill strings, and GB-A-2357527 discusses an apparatus for creating an acoustic wave signal in a well bore.
Due to the periodic structure of the drill string, which is typically formed from approximately 9.5 m lengths of drill pipe, wave transmission in certain frequency ranges (known as stop bands) is suppressed. This leaves distinct frequency ranges (known as pass bands) that can be employed for data communication, although there is also fine structure within the pass bands. Suitable carrier frequencies for torsional waves will probably be in the first pass band, which for standard drill pipes is around 250 Hz. However frequencies in the base band (around 0-140 Hz), or the second pass band (around 350-400 Hz), may also be suitable, depending on noise levels, attenuation and transmitter powers.
An important consideration for the realisation of practical acoustic drill string telemetry systems is the suppression of acoustic noise in the drill string so that at the acoustic receiver a high signal-to-noise ratio (i.e. the ratio of the power of the signal to the power of the noise) and hence high data transmission rates can be achieved.
For example, GB-A-2327957 discloses a noise isolating section which is introduced in the drill string e.g. to insulate an MWD sensor or transmitter from acoustic noise generated by the drill bit.
U.S. Pat. No. 5,128,901, on the other hand, is concerned with suppressing echoes in the drill string resulting from previously transmitted acoustic waves.
U.S. Pat. No. 4,066,995 discloses isolation subs which serve to attenuate vibrations in the drill string caused by operation of the drill bit and rotation of the rotating table on the drill platform. The isolation subs dissipate low-frequency vibration energy so that vibrational resonances can be prevented.